A variety of cationic catalysts are known to produce polymers out of olefin monomers in living and nonliving polymerizations. In a living polymerization, each initiator molecule initiates a growing polymer chain that does not undergo chain transfer or termination reactions while monomer is present. In some of these systems once the monomer is depleted the growing polymer chain undergoes an irreversible termination reaction. In theory, though, if the chains did not terminate then addition of monomer would cause the polymer chain to grow regardless of the length of time between monomer additions. In reality however, this reversible termination has been difficult to produce as later monomer additions grow unevenly to produce a broad molecular weight distribution. Such a distribution is the hallmark of a non-living system. Thus a living and terminationless system has heretofore not been strictly attained. It is an object of this invention to provide such a terminationless living system.
Typical cationic catalysts such as titanium tetrachloride (TiCl.sub.4), boron trichloride (BCl.sub.3), tin tetrachloride(SnCl.sub.4), iron trichloride (FeCl.sub.3), aluminum trichloride (AlCl.sub.3) systems and the like have been described in U.S. Pat. Nos. 4,910,321 and 4,929,683, and European Patent Application 341 012 for use in the living polymerization of olefins. The basic components of these systems include a Lewis acid, a tertiary alkyl initiator molecule containing a halogen, ester, ether, acid or alcohol group and an electron donor molecule such as ethyl acetate. The exact combination of the elements varies with each system. The tertiary alkyl initiators used in these systems are used for living and non-living carbocationic catalysts. The tertiary alkyl initiators are typically represented by the formula: ##STR1## wherein R.sub.1, R.sub.2, and R.sub.3 are a variety of alkyl or aromatic groups or combinations thereof, n is the number of initiator molecules, typically 1 to 6, and X is the functional group which the Lewis acid ionizes to bring about the carbocationic initiating site. This group is typically a halogen, ester, ether, alcohol or acid group depending on the Lewis acid employed. One or two functional groups per initiator tend to lead to linear polymers while three or more tend to lead to star polymers.
As discussed in U.S. Pat. No. 5,169,914, the chosen electron pair donor component of these systems directly relates to the ability of these catalysts to stabilize the carbocation formed and to generate living conditions. Electron pair donors have been defined as molecules capable of donating electron density to an electron deficient site. These molecules usually contain heteroatoms and heteroatomic functional groups including amides, ester, ethers, sulfoxides and the like. The electron donor number concept has been used to explain the activity of early catalyst systems which employ ether and ester initiators. It was believed that the formation of in situ electron pair donors were responsible for the catalyst characteristics. However, the role of the electron donor is still uncertain and has been challenged. See M Gyor, H. C. Wang, R. Faust, J. Macromol. Sci. A29, 639 (1992).
Catalyst systems based on boron trichloride and titanium tetrachloride using various combinations of the above components typically have similar process characteristics. First, Lewis acid concentrations must exceed the concentration of initiator sites by 16 to 40 times in order to achieve 100 percent conversion in 30 minutes (based upon a degree of polymerization equal to 890) at -75.degree. to -80.degree. C. These catalyst systems are also typically used with solvents. For example, the references above disclose methyl chloride as a preferred solvent and that a mixed solvent may be used to avoid side reactions or to keep the polymer in solution. Further the mixed solvent should provide some degree of polarity to maintain the polymerization rate. However, even in these circumstances, an electron pair donor must be present.
For an industrially applicable process these catalysts and polymerization conditions fall short of commercial usefulness. Improvements in these systems would include elimination of boron and titanium based Lewis acids as they present handling and purification problems. In addition, living systems also tend to require expensive proton scavengers to suppress initiation from unwanted protons, such as those from water contamination. In a preferred embodiment, this invention also provides a system that achieves living polymerization in the absence of a proton scavenger or an electron donor pair.